Polyoxymethylenes (POM), also known as polyacetals, are (co)polymers of formaldehyde. Pure formaldehyde is readily polymerized, but the polymer equally readily, spontaneously de-polymerizes, that is, the polymer unzips. Stabilization of polyoxymethylene can be achieved by end-capping the terminal hydroxyl groups with alkyl or acyl groups, by etherification or esterification respectively, as shown in Scheme 1. S. D. Nagare, U.S. Pat. No. 2,998,409, Aug. 29, 1961 to E. I. duPont de Nemours & Co. End-capped polyoxymethylene is sold commercially under the DELRIN™ trade name.
Stabilization of polyoxymethylene can also be achieved by copolymerizing formaldehyde with ethylene oxide, as shown in Scheme 2. C. Walling et al. U.S. Pat. No. 3,027,352, Mar. 27, 1962 to Celanese Corp. Polyoxymethylene copolymer is sold commercially under the CELCON™ trademark.
Polyoxymethylenes exhibit rigidity, high strength, excellent creep resistance, fatigue resistance, toughness, self-lubricity/wear resistance, and broad resistance to chemicals and solvents. Therefore polyoxymethylenes have utility in metal replacement. Metal items that can be replaced by polyoxymethylene include plumbing hardware, pumps, gears, and bearings.
Although polyoxymethylene is used in many applications, it has drawbacks which limit its use. Polyoxymethylene is a semi-crystalline plastic with a crystalline content of about 75 to 85%. Semi-crystalline plastics can become less rigid above their glass transition temperature and below their crystalline melting point. As a consequence, semi-crystalline plastics can exhibit a lower heat deflection temperature under high load than low load, for example, under a 1.82 versus a 0.455 megapascal load. Thus, it is desirable to increase the heat deflection temperature under high load for polyoxymethylenes. Polyoxymethylene, which has the lowest limiting oxygen index (LOI) of commercial thermoplastics—only about 15%—is extremely combustible and difficult to flame retard. H. Harashina et al. Polym. Degrad. and Stability, 91, (2006), 1996-2002. Z.-Y. Wang, Y. Liu, Q. Wang Polym. Degrad. and Stability, 95, (2010), 945-954. Thus there is a need for improvement in the flame resistance of polyoxymethylene. Q. Zhang et al. J. Polym. Res., 18 (2011), 293-303. Density is an important parameter for polymers. For an equal weight of polymer, more parts can be prepared from a polymer with a lower density. Polyoxymethylenes have a high density compared to other thermoplastics. Thus, a decrease in the density of polyoxymethylene compositions can have economic benefits.
In summary, there is a need in the art for polyoxymethylene compositions having higher heat deflection temperature under high load, higher flame resistance, and lower density.